Limited drug penetration is an obstacle that is often encountered in the treatment of CNS diseases. One mechanism that may contribute to this phenomenon is the expression of ATP-binding cassette (ABC) drug efflux transporters (i.e. P-glycoprotein or P-gp, Multi-drug resistance proteins or MRPs, breast cancer resistance protein or BCRP, a.k.a. ABCG2) at the blood brain barrier (BBB) and blood cerebrospinal fluid (BCSF) barrier. ABC transporters also localize to a lesser extent at the CNS parenchyma cells where they act as secondary barrier to neural penetration of substances. Efflux transporters also extrude catabolites and toxins to prevent their harmful accumulation in the cell, constituting the major mechanism of cell adaptation to disease-mediated and environmental stress. Little is known on ABC transporters localization and regulation in amyotrophic lateral sclerosis (ALS), a neurodegenerative disease of the motor system. Our preliminary data show increased P-gp expression in spinal cord astrocytes of the SOD1-G93A mouse model of ALS as well as in spinal cord specimen homogenates of sporadic and familial ALS patients. From a therapeutic perspective, this suggests that the obstacle to drug penetration in the CNS is increased by the disease and must be overcome to develop effective pharmacotherapies for ALS. Given their multi-specificity, the recognition of efflux transporters as critical players in CNS diseases is unquestioned although important questions remain unanswered. For example: How does ALS affect efflux transporters localization and function? Which ALS-specific signaling pathways are responsible for up-regulation in P-gp? Will ALS-mediated up-regulation in P-gp and/or other ABC transporters change how we therapeutically treat the mouse model of the disease, and ultimately ALS patients? To fill this gap in knowledge, we propose: (1) To investigate activity, expression and distribution profile of P- gp and other relevant ABC drug transporters in ALS; (2) To study whether efflux transporter activity in non- neuronal cells contributes to motor neuron degeneration in in-vitro and in-vivo models of ALS; (3) To investigate the impact of eliminating ABC transporter function on ALS therapeutics.